1. Field of the Invention
This invention relates generally to Roots blowers suitable to use for aeration in river-basin sewerage or small waste water disposal facilities, transportation of powder, etc., and more particularly to such a Roots blower in which a noise produced during its operation can be reduced.
2. Description of the Prior Art
Conventional Roots blowers of the positive displacement type comprise a casing formed to have a generally oval cross section and having an inlet port and an outlet port, and a pair of rotors mounted on parallel shafts in the casing to be rotated in opposite directions. Each rotor has a multilobed structure. Upon rotation of the rotors, chambers are defined by an inner peripheral wall surface of the casing and lobes of the rotors respectively. It is known in the art that in the Roots blowers of the above-described type, supercharged suction air flows backward from the outlet port side to each chamber through a small clearance between each rotor and the casing at an initial stage of delivery stroke. It is also known that such a backflow of the suction air produces noise. To reduce the noise due to the backflow, the assignee of the present application proposed, in Japanese Utility Model Publication No. 49-63507 published on Jun. 4, 1974, a Roots blower provided with straight grooves formed on an inner peripheral wall surface of the casing to extend along a rotation direction of each rotor. The assignee further proposed, in Japanese Patent Publication No. 3-124986 published on May 28, 1991, another Roots blower provided with helical grooves formed on the inner peripheral wall surface of the casing at a predetermined angle to an imaginary line parallel with each rotor axis.
Furthermore, Japanese Utility Model Publication No. 62-173579 published on Nov. 4, 1987 discloses a supercharger comprising a rotor housing having an inlet port and an outlet port, and a pair of two-lobed rotors rotatably mounted on respective shafts in the rotor housing. The rotor housing has two sets of straight grooves each of which sets includes a plurality of straight grooves formed on the inner peripheral wall surface of the casing to be parallel with axes of the rotors. The straight grooves of each set are formed to extend from a location on the inner peripheral wall surface of the casing corresponding to an apex of a front lobe with respect to the rotation direction of each rotor to the outlet port when communication between the inlet port and a chamber defined by the front lobe, rear lobe and the inner peripheral wall surface of the casing is cut off by the rear lobe with respect to the rotation direction of the rotor, whereby noise reduction is improved.
However, the noise reduction provided by each of the above-described Roots blowers and supercharger is insufficient. Further noise reduction has been desired for an improvement in residential or workshop environment. Furthermore, the Roots blower needs to be of small size and to have a high-speed rotation structure from the viewpoint of its production cost. However, since this structure has resulted in an increase in the operation noise, the noise reduction has technically been difficult in the prior art.